What size would you recommend?
2-3 years is pretty good.

Hornbeam grows 2-3ft per year, so it depends on how high you want your
hedge. Hornbeam is ideal in wet clay conditions, whereas Beech prefer
well-draining soil.

When pruning a new hedge, don't cut the leading branch (with the exception
of hawthorn) until it has reached the desired height, in early autumn cut
the sides of the trees to encourage a dense hedge.

HTH

Cheers

Nick
http://www.ukgardening.co.uk